Coordinated Universal Time

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Coordinated Universal Time (abbreviated UTC)[1] is the time standard by which the world regulates clocks and time. Computer servers, online services and other entities that rely on having a universally accepted time use "Coordinated Universal Time" for that purpose. Though there are scientific differences (explained below) between "Coordinated Universal Time" and the time standard known as Greenwich Mean Time (GMT) - in the very broadest sense as understood by non-scientists, "Coordinated Universal Time" in essence is Greenwich Mean Time (GMT). When times are listed as being "UTC" - this is de facto the equivalent of "GMT".

Scientifically speaking, "Coordinated Universal Time" is a time standard based on International Atomic Time (TAI) with leap seconds added at irregular intervals to compensate for the Earth's slowing rotation.[2] Leap seconds are used to allow UTC to closely track UT1, which is mean solar time at the Royal Observatory, Greenwich.

Since the difference between UTC and UT1 is not allowed to exceed 0.9 seconds, if high precision is not required, the general term Universal Time (UT) may be used.[3]

In casual use, when fractions of a second are not important, Greenwich Mean Time (GMT) can be considered equivalent to UTC or UT1. Saying "GMT" often implies either UTC or UT1 when used within informal or casual contexts. In technical contexts, usage of "GMT" is avoided; the unambiguous terminology "UTC" or "UT1" is preferred.[3]

Time zones around the world can be expressed as positive or negative offsets from UTC as in this list; UTC replaced GMT as the basis for the main reference time scale or civil time in various regions on 1 January 1972.[4]



UTC is the time standard used for many Internet and World Wide Web standards. In particular, the Network Time Protocol, which is designed to synchronise the clocks of many computers over the Internet (usually to that of a known accurate atomic clock), uses UTC.

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